How to access an objects DupliGroup instances from Python?
You need to create a dupli-list. eg:

def object_derived_get(ob, scene):
    if ob.dupli_type != 'NONE'
        ob.dupli_list_create(scene)
        ob_matrix_pairs = [(dob.object, dob.matrix.copy()) for dob in ob.dupli_list]
        ob.dupli_list_clear()
    else:
        ob_matrix_pairs = [(ob, ob.matrix_world.copy())]

    return ob_matrix_pairs

# example
import bpy
from bpy import context
for ob, matrix in dupli_list_create(context.object, context.scene):
    print(ob, matrix)


Seealso:


http://www.blender.org/api/blender_python_api_2_74_release/bpy.types.DupliObject.html
http://www.blender.org/api/blender_python_api_2_74_release/bpy.types.Object.html#bpy.types.Object.dupli_list